Currently, a battery pack generally comprises a number of battery units connected in series. Because of differences among the individual battery units in terms of the charging status, the impedance, the temperature characteristics or the like, an imbalance among the individual battery units may arise. Such an imbalance will lead to a reduced capacity and a shortened service life of the overall battery pack. Therefore, it is necessary to use a battery equalizing circuit in the battery pack to compensate for the imbalance so as to maintain the capacity and prolong the service life of the battery pack.
FIG. 1 is a schematic view of a prior art battery equalizing circuit. As shown in FIG. 1, the prior art battery equalizing circuit 100 is an active equalizing circuit designed on the basis of an inductor, and comprises a battery unit 110, a battery unit 120, an inductor 130, a control switch 140 and a control switch 150. The battery unit 110, the inductor 130 and the control switch 140 form a first circuit loop, and the battery unit 120, the inductor 130 and the control switch 150 form a second circuit loop. The control switch 140 and the control switch 150 are controlled to be turned on and off alternately by a pair of complementary pulse-width modulation (PWM) signals. By adjusting duty ratios of the PWM signals, a magnitude of the equalizing current can be adjusted so that, through the inductor 130, energy from the battery unit 110 is transferred to the battery pack 120 or vice versa.
The prior art battery equalizing circuit 100 features a simple structure and can supply a large equalizing current. However, the equalizing current is liable to influences from many factors such as temperature changes, changes in voltage levels of the battery units, differences among individual devices, the line resistances and so on. Therefore, the equalizing current may vary greatly even if the duty ratios of the PWM signals remain unchanged, which makes the equalizing effect of the whole circuit unsatisfactory. Furthermore, the equalizing current in the prior art battery equalizing circuit 100 may exceed ratings of devices in some cases to cause risks. Accordingly, an urgent need exists in the art to develop a novel battery equalizing circuit that can solve the aforesaid problems.